Field of the Invention: This invention relates to aqueous, base-solubilized resinous dispersions of acrylic polymers in combination with epoxy resins. More particularly, this invention relates to coating metal containers such as containers for alcoholic beverages with these aqueous dispersions.
Brief Description of the Prior Art: In the manufacture of metal containers for food and beverages, a resinous coating is applied to the interior of the container. A coating is necessary because the bare metal can affect the taste or even ruin the contents of the container. To be effective, the coating must be as continuous and as void free as possible and, of course, must also not affect the taste of the container contents itself. Many of the prior art container coating compositions are organic solvent based. However, because of environmental problems these coatings create, they are being replaced by aqueous-based coating compositions.
Examples of water-based container coatings are those described in U.S. Pat. Nos. 3,943,187 and 3,997,694 to Wu. These compositions are based on amine solubilized acrylic polymers in combination with epoxy resins. The acrylic interpolymers are prepared by solution polymerization techniques in alcoholic solvents and contain from about 0.5 to 10 percent by weight of acrylic acid and 1 to 10 percent by weight of a higher alkoxymethylacrylamide or methacrylamide such as N-butoxymethylacrylamide. Although such compositions are probably useful in many container coating applications, we believe that there are a number of disadvantages associated with such compositions. First of all, the dispersions do not have long-term stability. Although the acrylic acid content can be as high as 10 percent, it has been found that aqueous coating compositions of the acrylic-epoxy type in which the acrylic polymer is prepared with higher levels of acrylic acid or a comparable acid, that is, about 15 to 85 percent and preferably from about 25 to 70 percent, and more preferably 30 to 60 percent, are more compatible with the epoxy resin component resulting in more dispersible compositions which have better curing properties.
Also, acrylic polymers prepared with N-butoxymethylacrylamide may not be completely acceptable for use in coating compositions for interior liners of alcoholic beverage containers such as beer cans especially if undercured. Such acrylic polymers contain residual butanol in a combined form which could furnish free butanol upon hydrolysis. Butanol, even in minute concentrations, can adversely affect the taste of the beer. Also, the acrylic polymers of the aforementioned prior art are prepared by solution polymerization techniques in the presence of primary and secondary alcoholic solvents such as butanol and isopropanol. Thus, even if the acrylic polymers were prepared with N-ethoxymethylacrylamide or methacrylamide moieties such as required by the present invention, there is a danger of some transetherification in the acrylic polymerization step which could result in residual amounts of the alcoholic solvent such as butanol and isopropanol combined with the acrylic polymer which, as explained above, could adversely affect the taste of the beer.